Method for the reproduction of color



United States Patent 3,337,340 METHOD FOR THE REPRODUCTION OF COLOR Josef Matkan, Malvern, South Australia, Australia, as-

signor to Research Laboratories of Australia Limited, North Adelaide, South Australia, Australia No Drawing. Filed Nov. 19, 1962, Scr. No. 238,786 Claims priority, application Australia, Dec. 28, 1961,

1 Claim. (Cl. 96-1) This invention relates to reproduction of color by electrophotography, and particularly to the reproduction of color in such forms as to be substantially similar in hue, balance and intensity to that produced by lithographic and other mechanical printing methods and color photography.

It is known that electrophotographic images can be produced in any desired color by one skilled in the art, and toner formulations capable of producing images of various colors are known. It is also known to produce such images in a multiplicity of colors by the use of repetition exposure and development procedures on the same electrophotographic sheet.

Color printing by xerographic methods usually involves the use of a photosensitive material on a relatively conducting backing or supporting layer such as a paper or metal sheet or film, the photo-sensitive layer comprising for instance a particulate photoconductor such as zinc oxide or the like in a resin binder, such coating resin being sufficiently hardened or poiymerised that its electrical properties are not affected by environmental changes during production of the print. Such a sheet is first sensitized by applying an electrical charge thereto. This may be accomplished, for example, by subjecting it to a corona discharge from a series of wires or points held at a relatively high direct current potential above a base on which the sheet is placed. The polarity should be such as to impart a negative potential to the photo-layer in case such photo-layer is composed of zinc oxide since this material is not capable of supporting a positive electrostatic charge. The sheet is then subjected to a light pattern corresponding to the requirements of the first color to be applied, and after this exposure the sheet is developed in a dish or bath or by means of some other device containing the liquid toner dispersion of the appropriate color. It is preferable to dry the sheet after this development step as this allows the first deposited image to become sufiiciently adherent to the photosurface to enable it to remain in position during subsequent handling.

Subsequent colors are deposited in turn by repetition of the charge, expose, develop and dry cycle, the exposure step in each instance imposing the correct light pattern corresponding with the particular color being reproduced at that time. Provided the successive images are placed in register with each other using a balanced set of compatible, overprinting toners, the resultant image will be substantially similar to that which would be produced by a mechanical printing using the same separations and inks corresponding in hue to the toners used.

A problem however in overprinting xerographically is that the developers tend to affect the charging characteristic and bleeding away of the charge on the photoconductor surface and therefore while the first color can be put down in the normal manner, this color can then modify the charge which different areas can take when preparing the photoconductor for the application of the second color, the problem being intensified when an attempt is made to charge through the first two colors in preparation for the application of a third color.

The problem which is thus raised is that while overprinting by mechanical means is a simple matter as each 3,337,340 Patented Aug. 22, 1967 block applies the ink in proportion to the ink carried by the block surface, in xerography the electrostatic pattern itself is varied by the application of the colors and therefore unless complete overprinting is obtained, the result is not a true representation of the image which is projected onto the surface in each of the succeeding stages.

The object of this invention is to provide a form of developer which will be as close as is reasonably possible to mechanical overprinting, and we have found that this can be achieved by closely regulating the quantity of insulating matter such as resin which is used in conjunction with the pigments in the developer, it having been found by us that if there is sufiicient of such insulating resin present to just cover the pigment surface, the insulating surface formed on the developed image will be capable of supporting a charge, while the proximity of the photoconductor to the surface allows the charge to be bled away under the influence of light.

In this way it is possible to eliminate any electrical modification which may otherwise have been introduced by the preceding developer and to produce an image which is a true electrostatic representation of the negative from which the image was produced, and therefore when this is subjected to a developer, a true overprint results.

We have found that some of the highly satisfactory resins for this purpose are certain pentaerythritol, epoxy ester, hydrogenated rosin, alkyd and other synthetic resins of appropriate electrical insulating properties, such as for instance a volume resistivity greater than 10 ohm. cm. and a dielectric constant preferably less than 3.5, such resins being used in a quantity which is so balanced in relation to the pigment that sufficient resin exists in excess of the quantity required to wet the pigment and to fill the voids between the deposited particles to form an insulating film at the pigment surface or alternatively such required resin film may be subsequently placed thereon to be then capable of supporting an electrostatic charge but not to the extent that such charge could not be bled away when the photoconductor surface beneath the resin is subjected to light.

The quantities and proportions will be appreciated from the examples which form part of this specification.

This present invention therefore embodies the method and means for the production and use of 'toners of various colors capable of simulating the hue, intensity transparency, and overprinting characteristics of lithographic, gravure and letterpress inks, when such toners are used under controlled conditions as herein defined.

The basic features of the invention will be apparent from the following description.

To obtain overprinting the image areas must contain an insulating component either within the image forming pigment structure or as a layer over the image, such component being capable of supporting an electrostatic charge in case the surface is recharged for the subsequent color step. This is especially the case with pigments which are so much conductive that the photoconductive layer beneath then cannot become charged. In such cases the said insulating component which is preferably a resin (termed from now on overprinting resin) is the only medium capable of supporting a charge if the image is re-charged with the purpose of depositing another color which has to overlap the previous color.

An image formed by a conductive pigment can be rendered sufliciently insulating so as to be capable of supporting a charge by the following methods:

(a) Incorporating overprinting resin with pigment, so that resin is in suspension and co-deposits with the pigment. The resin must be of the insulating type and must not be ground on to the pigment particles as a thin control layer of relatively conductive type as taught in prior art.

The overprinting resin must be present with the pigment in dispersed form in a quantity such that there is suffi cient resin for adequate charge holding.

The co-deposited overprinting resin acts as a binder for the pigment, fills the image structure and acts as key for further overprinting resin quantities which may be deposited later in some other form.

(b) The overprinting resin can be deposited over the conductive image by dipping the print containing the conductive image in a solution of the overprinting resin in a non-polar solvent of adequate insulating properties such as for instance a volume resistivity greater than 10 ohm. cm. and a dielectric constant preferably less than 3.0. The image is developed in a liquid containing only dispersed pigment or pigment and some dispersing medium and then it is removed from the developer liquid and placed into the overprinting resin solution. There is still some charge left on the image areas so that in addition to an overall cover, preferential resin deposition takes place in image areas.

(c) The overprinting resin can be deposited by dipping the print containing the conducting image in an overprinting resin solution in order to obtain an overall resin coating. For this purpose the image is developed in a vehicle containing the pigment dispersion or the pigment and some dispersing medium, the print is dried after developing and then dipped in the overprinting resin solution.

The concentration of overprinting resin solution in (b) and (c) must be such that the resin layer over non-image areas is not excessive so that no noticeable increase of charge holding capacity with consequent decrease in photoconductivity of the zinc oxide is caused in the imagefree or background areas. In addition, the deposited layer over the image areas must be capable of supporting a charge over the conductive pigment structure yet it must also be capable of permitting such charge to leak away when the zinc oxide layer underneath the image is exposed to light in areas where no overprint is desired.

In view of this it is preferred to have a weak overprinting resin solution and to dip the print several times thereby positively controlling the optimum thickness of overprinting resin layer. It is preferable to dry the print between each dip.

A smooth overprinting resin overall coverage can be obtained in (b) and (c) if to the overprinting resin solution a weaker solvent is added which acts as a diluent and promotes solvent release by a fast rate of evaporation, or if a weaker solvent is added which acts as a differential solvent causing resin precipitation upon evaporation of the true solvent, such weak solvent having a slower rate of evaporation than the true solvent.

It is quite obvious that in cases (b) and (c) the resin layer can be applied by means other than dipping, such as by spraying, by electrostatic deposition, by wet roller, in form of a contact adhesive, from a liquid wedge applicator, etcetera.

The process as outlined in (b) and can be employed after each color deposition whenever a conductive pigment is used, but depending on the colors and pigments used in the process, only one dip may be sufficient.

The overprinting resin used in (b) and (c) must of course be compatible with the dispersing medium used for the pigment in the developing vehicle and it must be compatible with any different overprinting resin used with preceding or subsequent colors.

The overprinting resin should be of fast drying type and of fast solvent releasing type. In dried form it should not be attacked, swelled or removed by the solvent action of the subsequent vehicles. When dried this overprinting resin should have such electrical properties as hereinbefore described that it is capable of supporting a charge adequate to attract pigment or other particulate matter for the formation of images of the desired intensity.

It follows that any vehicle in which an image rendered insulating by such overprinting resin is to be developed, should be weaker in solvent power than the liquid in which the overprinting resin is dissolved, whether or not such overprinting resin solution contains a diluent or a differential solvent, and in view of this, the liquid vehicles should be progressively weaker in solvent power, or at least all vehicles subsequently applied to an image containing an overprinting resin insulator should be of such solvent power that the overprinting resin coating previously applied is not softened by such subsequently applied liquid.

This invention is preferably based on the following sequence of colors: blue, red, black and yellow, and in view of this, the formulations appearing herein and the process as described is arranged for the above sequence but this is not to limit the invention in any way as it is possible to adjust the resin solution to allow a different color sequence.

The following is a detailed description of certain illustrative forms of the method by which a four color electrophotographic image can be produced using the embodiments of this invention.

An electrophotographic sheet carrying a surface coating of resin-held photoconductive particles is prepared by any known method.

It is important however, that the resin which anchors the photoconductors should be impervious to and resistant to solution in the liquid used for purposes of development and washing. The resin may be made resistant by prolonged curing at temperatures such as P. which do not char the paper on which the coating is placed or if the backing is of metal or other heat-resistant material then curing may be carried out in a shorter time at the recommended curing temperature for the resin. The following photoconductive coating can be used in this process:

Grams Zinc oxide durham special Z 900 Rhodene M8/50 600 Toluol 250 Manganese naphthenate (4%) 2.5 Cobalt naphthenate (3%) 2.5

The ingredients can be milled together, diluted if required, and then deposited on a relatively conducting support by any known coating method.

Example I This example illustrates the first embodiment of this invention where a relatively conducting pigment is dispersed in a liquid vehicle containing the overprinting resin in solution. Four toners are prepared as follows:

(1) Blue devel0per.25 grams of the pigment phthalocyanine blue such as Irgalite blue, having a mean particle diameter of less than 0.5 micron, are ground together with 100 grams of a resin such as Beckosol Solution 1352 so as to wet the pigment with the resin. This mixture is then dispersed in 1000 milliliters of Shell solvent X55. This solution is then allowed to stand for not less than three hours in order to ensure stability, particularly with regard to adsorption of the resin by the pigment. The developer is then ready to use.

(2) Red developer.100 grams of the pigment Para red, or Toluidine red or Lithol red, having a mean particle diameter of approximately 0.1 micron, is dispersed with grams of Beckosol Solution 1352 in for instance Shell solvent X55 in a similar way to the blue pigment.

(3) Yellow devel0per.-100 grams of an organic yellow pigment such as Benzidine yellow or Hansa Yellow or Monolite yellow are dispersed with 150 grams of Beckosol Solution 1352 in for instance Shell solvent X55 in a similar way to the blue pigment.

(4) Black devel0per.95 grams of carbon black such as the pigment Peerless Black and 5 grams of Reflex Blue in Varnish No. 69884 are dispersed with 400 grams of Beckosol Solution 1352 in for instance Shell solvent X55 in a similar way to the blue pigment.

These four developers are now used in the following manner:

The electrophotographic sheet is subjected to a corona discharge to produce on it a uniform electrostatic charge of the order of 200 to 400 volts. The charged sheet is subjected to exposure to an image produced by filtering the colored image with an orange filter or by contact printing from a black and white separation transparency as it is well known to do in the printing industry, in this case to the blue separation. This image is then developed in a dish of blue developer for 30 seconds and then washed in a poor solvent such as Shell X4 solvent or in iso-octane, or Freon 113. The image so obtained is then dried oif by subjecting it to an air blast or by suspending it in the air for 2 minutes, when the ambient temperature is 70 F. or higher, in order that such dried resin film should be rendered sufiiciently insoluble in the vehicle used for subsequent colors.

The sheet is then re-charged by once again exposing it to the corona discharge, producing on its surface another charge which resides over the whole of the previous image.

The sheet is then exposed to a green filtered image or to a red separation, and the result is developed in the red developer of Example 1. The image is then washed and dried as previously described for the blue image.

The process is repeated for the black and then the yellow images. Example II This example illustrates the second embodiment of this invention (Case b) where an overprinting resin layer is deposited by placing the image after color developing into the overprinting resin solution. Four toner concentrates containing pigments and dispersing media are prepared as follows: a (I Blue toner concentrate Grams Heliogen Blue G pigment 20 Magnesium carbonate l0 Paralac RPlO resin 80 Cold blend ingredients and mill. Magnesium carbonate has been incorporated to obtain a certain color value.

(2) Red toner concentrate Grams Brillfast Rose Red pigment No. 4444 20 Brillfast Geranium pigment N0. 3543 20 Calcium resinate varnish in bodied linseed oil (25% resin) 50 Cold blend varnish with pigments and mill.

(3) Black toner concentrate Grams Carbon Black SRF 40 Reflex Blue in Varnish No. 69884 4 Superbeckacite 2100 resin varnish in bodied linseed oil (20% resin) 100 15% Lead naphthenate (in hydrocarbon solvent) 50 50% Calcium resinate solution in Solvesso 100 200 Cold blend ingredients and mill.

(4) Yellow toner concentrate Grams Monolite Yellow GNS 100 Bodied linseed oil 150 Cold blend ingredients and mill.

concentrate is dispersed in a solution of the resin Pentacite P423 in cyclohexane in the proportion 5% of Pentacite P423 resin in the cyclohexane. After the blue image has been developed it is washed in a liquid comprising one part of 5% Pentacite P423 resin solution in cyclohexane to one part of Shell solvent X55.

The overall solvent power of this rinsing liquid is smaller than that of the liquid vehicle in which the blue image has been developed. The red concentrate is dispersed in Shell solvent X55 and is used for development in this form but the rinsing operation after development is carried out in a liquid comprising one part of 5% Pentacite P423 resin solution in cyclohexane to two parts of Shell solvent X55. The black development step is carried out in a dispersion of the black toner concentrate in Shell solvent X55 and the rinsing operation is carried out in the same manner as for the red color. The yellow development step is carried out in a dispersion of the yellow toner concentrate in n-heptane or Shell solvent X4, both of which are poorer solvents than Shell solvent X55.

It will be seen that the invention aims to ensure that there is suflicient resin present to maintain the surface charge after each development and that as one particular embodiment of this invention lower solvent power liquids are used as vehicles for toners which have to be deposited to obtain overprinting effect on to previously deposited images because such lower solvent power liquids are not capable of attacking or dissolving the overprinting resin layer which carries the surface charge.

Example 111 This example illustrates the third embodiment of this invention (case c) where an overprinting resin layer is deposited by dipping or other means on to a developed and dried image.

In this embodiment of the invention, the four developers are made up merely by dispersing pigment in the desired carrier liquid with the aid of a wetting agent such as iso-propyl alcohol, oleic acid, cyclohexane, stearic acid, lauric acid, myristic acid or the like or other well known methods of dispersing pigments in solvents of adequate volume resistivity used in the paint and ink industries. The invention consists of developing the colors in the sequence and manner already described but the images are dried after each development. and then dipped in or contacted by some other means with a solution of resin in a suitable solvent. It is important to distinguish between having the resin in suspension and having it in solution. Examples of the resin solutions which may be used in this embodiment of the invention are as follows:

(1) A solution of the resin, Silicone 804, in iso-octane in the proportions 5 percent of the resin by weight.

(2) A solution of the hard synthetic resin Pentarol 1, in cyclohexane in the proportion 3 percent by weight of the resin. To prevent the possibility of formation of streaks in drying, to this solution is added 10 percent by volume of Shell solvent X55.

(3) A solution of the resin Epidene E168, in perchlor- ,ethylene, to the extent of 3 percent by weight of resin.

(4) A solution of the resin Vandene M66 dissolved in cyclohexane in the proportion 5 percent by weight of resin.

In this embodiment it is believed that the dry pigments dispersed in the carrier liquid and used to form an image then selectively adsorb resin from the resin solution and so enhance charge holding capacity.

A convenient method of assessing the efiiciency of the resin solutions and their effect on overprinting is to dip one half of a sheet of the electrophotographic material in the solution and dry it as would be done when a picture is being produced. This dipped and dried sheet is then charged and exposed to a convenient pattern which covers both the dipped and undipped areas. The result is developed and the difference between the areas is noted.

There should be no difference between the two areas if the resin film is intended not to impair overprinting. It is also possible to assess the electrical effect of the dipped resin by comparing by electrometric means the potential difference or difference between the surface charge on the dipped and undipped areas. The dipped areas will tend to retain from 20 to 50 percent more charge than the undipped areas in the dark but the light decay rate is for practical purposes the same for both areas.

The following are descriptions of materials which have been referred to.

Irgalite Blue pigment-manufactured by Geigy Company,

Switzerland.

Heliogen Blue pigment-manufactured by Badische Anilin & Soda Fabriken, Germany.

Brillfast Rose Red No. 4444 and Brillfast Geranium No. 3543 pigmentsmanufactured by Smith Reichold Colour, England.

Monolite Yellow pigment-manufactured by I.C.I., England.

Peerless Black-a carbon black pigmentmanufactured by Columbian Carbon Company, New York, USA.

Carbon Black SRF pigment-manufactured by Australian Carbon Black Pty. Limited.

Reflex Blue in Varnish No. 69884, tinting ink for black manufactured by Reichold Chemicals, U.S.A.

Rhodene M8/50is an isophthalic short oil alkyd resin based on linseed oil, 40% oil length acid value 10, specific gravity .9870.997.

Beckosol Solution 1352is an isophthalic long oil alkyd resin based on semi-drying oil, 60% oil length, acid value 3-6, specific gravity 0.940 (solution) dissolved in Mineral Spirits, 60% non-volatile matter.

Paralac RPLO-is a long oil alkyd resin based on linseed oil, acid value 512, specific gravity 1.034.

Pentacite P-423-is a pentaeryth-ritol resin, melting range 121132 C., acid value 20-30, specific gravity 1.05 1.15.

Silicone Resin 804-is a silicone protective coating resin, 60% resin solids dissolved in toluene, specific gravity 1.05-1.07.

Epidene E168-is an epoxy ester resin based on tall oil 50% oil length, acid value 6, supplied as a 50% solution in an aromatic solvent (such as toluene), specific gravity 0.96-0.97 (solution).

Superbeckacite 2100is a pure phenolic oil reactive resin, melting range 129146 C., specific gravity 1.07, 100% non-volatile matter.

Pentarol 1-is an unmodified pentaerythritol ester of rosin,

melting range 8090 C., acid value 7-15, specific gravity 1.08, 100% non-volatile matter.

Vandene M 66is a resin modified maleic resin, melting range 100115 0, acid value -20, specific gravity 1.10, 100% non-volatile matter.

' Freon 113, trichlorotrifluoroethanemanufactured by Du Pont, USA.

8 Solvesso 100, hydrocarbon solvent, boiling range 174 C., KB value 93, Standard Oil Company. Shell X55, petroleum fraction, boiling range 58-140 0.,

KB value 40, Shell Company.

Shell X4, petroleum fraction, boiling range 5870 C.,

KB value 30, Shell Company. What I claim is: A wet method of developing multiple colors by the overprinting of multiple electrostatic images on a single electrophotographic sheet comprising:

producing a first image by charging and light-modifying a photoconductor surface on said sheet,

developing the first image with a liquid developer comprising an insulating carrier liquid in which are suspended conductive pigment particles of the first color and depositing on said particles an overprinting electrically insulating bonding medium in a liquid vehicle, the quantity of such bonding medium being sufficiently in excess of that adsorbed by the pigment and in excess of the voids between the pigment particles to produce a chargeable layer of bonding medium covering the pigment particles, producing a second image by charging said chargeable layer of said electrophotographic sheet including such layer of bonding medium and light-modifying such charge to produce the second electrostatic image,

producing a further but different colored layer of such pigment particles and bonding medium in a successive developed image by having such an excess of bonding medium in relation to the pigment, and charging and light-modifying the said electrophotographic sheet prior to each development, each bonding medium liquid vehicle for each developer sequence being substantially weaker in solvent power than the liquid vehicle in which the overprinting chargeable bonding medium being developed is contained,

whereby a plurality of images of different color may be developed successively on the photoconductor surface by charging the said insulating layer of each developer and any exposed photoconductor layer substantially uniformly before bleeding away the charge image-wise by exposure to light.

References Cited UNITED STATES PATENTS 2,907,674 10/ 1954 Metcalfe et al 961 3,057,720 10/ 1962 Hayford et al 961 3,060,020 10/1962 Greig 961 3,076,722 2/1963 Greig 961 3,081,263 3/1963 Metcalfe et al 252621 3,241,957 3/1966 Fauser et al 961 NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner. 

